You're crazy - that will do rude things to your spring rates!
I don't have the formulae handy, but they should be findable with a little Googling. They essentially come down to, as I recall, a function of the spring wire diameter and the number of coils - that's a large part of why ricers with cut springs bounce around so damn much.
Honestly, you'd probably be better off getting springs wound to order than trying to fab something like that up - you want to be safe, don't you?
How high are you trying to go, anyhow? If you're thinking up fabrication ideas like that, the idea itself may be just a little outlandish...
5-90